This can happen if someone has their browser set to reopen their tabs from a previous browsing session whenever they open up their browser again. If a viewer leaves your document open in one of those tabs, then you'll see a 0-duration visit each time they reopen the browser.
It's also possible, although much rarer, to have this happen when a viewer happens to double-click a link you've sent them, opening your document as well as in a background tab in their browser. If they subsequently close their browser before going into that background tab and actually viewing the document – because they've been reading the document from the other tab they've opened – then this will count as a visit, although they technically wouldn't have looked at the backgrounded browser tab.
In both cases, we've opted to show you that your content has been opened by a viewer.